Abstract: Melatonin (N-acetyl-5-methoxytryptamine, MT), a small-molecule indoleamine widely distributed in plants and animals, plays an important role in regulating plant growth, development, and stress resistance. Using CiteSpace software, this study conducted a visual analysis of domestic and international literature published from 2015 to 2025, revealing a rapid increase in research on the role of exogenous melatonin in regulating crop quality. Accordingly, the research progress in this field is systematically reviewed from the perspectives of biosynthesis, quality regulation, underlying mechanisms, and stress resistance-mediated quality improvement. As an exogenous signaling molecule, melatonin can stimulate endogenous MT synthesis, and its content varies dynamically in response to environmental conditions and plant developmental stages. Its role in quality regulation is particularly evident during fruit maturation. Specifically, exogenous melatonin improves external quality attributes, such as maintaining fruit color and firmness, while enhancing internal quality by increasing the contents of nutrients, such as soluble solids and vitamin C, as well as volatile aromatic compounds. These effects are primarily attributed to the ability of melatonin to maintain crop quality during postharvest ripening and senescence by inhibiting ethylene biosynthesis, activating the antioxidant system, and maintaining energy metabolism. Furthermore, melatonin plays an important role in maintaining crop quality under adverse environmental conditions. It enhances fruit cold tolerance by inducing proline accumulation and regulating membrane lipid metabolism; alleviates heavy metal and sodium ion toxicity by maintaining osmotic balance and ion homeostasis; and improves disease resistance by activating jasmonic acid and salicylic acid signaling pathways and promoting the expression of pathogenesis-related proteins. Based on the current research status, future studies should focus on the following three aspects: 1) elucidating the broad-spectrum mechanisms underlying the synergistic effects of MT on quality and yield improvement and stress resistance-mediated quality enhancement; 2) expanding research on the field application of MT; and 3) investigating the combined effects and synergistic interactions of MT with other regulatory substances. These efforts will provide new theoretical foundations and practical guidance for the regulation of crop quality by plant melatonin and its agricultural applications.